The proposed research evaluates the impact of several physiological stimuli on circannual rhythms of golden-mantled ground squirrels, including the role of endogenous circannual fluctuations in body temperature on circannual variations in circadian organization and metabolic rate. The decrease in body temperature normally associated with the hibernation season will be blocked and the presence of circannual variations in circadian entrainment and metabolic rate determined. The role of prenatal photoperiod in the timing/generation of circannual rhythms of young squirrels will also be examined in squirrels kept either in constant light, to eliminate photoperiod cues, or a simulated natural photoperiod during gestation. The timing and/or presence of circannual rhythms in the offspring of these females will be examined. Neural sites at which testosterone (T) suppresses hibernation will be examined in male squirrels implanted with cannulae containing T into neural sites that contain T receptors and implicated in thermoregulation. The inhibitory role of the suprachiasmatic nucleus (SCN) in circannual timing of hibernation and reproduction in male squirrels will be examined to determine if this nucleus functions similarly in males as in females to limit hibernation and reproduction to particular times of year. This research may help in the development of interventions for treating human disorders that have a circannual basis.